Computational and Experimental Study on Selective sp2/sp3 or Vinylic/Aryl Carbon-Hydrogen Bond Activation by Platinum(II): Geometries and Relative Stability of Isomeric Cycloplatinated Compounds

Article abstract of DOI:10.1021/acs.organomet.5b00326

Cyclometalating ligands 6-(1-phenylethyl)-2,2′-bipyridine (L4), 6-(1-phenylvinyl)-2,2′-bipyridine (L5), and 6-(prop-1-en-2-yl)-2,2′-bipyridine (L6) were synthesized by the Negishi coupling of 6-bromo-2,2′-bipyridine with the corresponding organozinc reagents. The reaction of L4 with K₂PtCl₄ produced only the cycloplatinated compound 4a via sp² C-H bond activation. The reactions of L5 and L6 produced exclusively the cycloplatinated compounds 5b and 6a, respectively, via vinylic C-H bond activation. DFT calculations were performed on 12 possible cycloplatination products from the reaction of N-alkyl-N-phenyl-2,2′-bipyridin-6-amine (alkyl = methyl (L1), ethyl (L2), and isopropyl (L3)) and L4-L6. The results show that compounds 1b-3b resulting from the sp³ C-H bond activation of L1-L3 are thermodynamic products, and their relative stability is attributed to the planar geometry that allows for a better conjugation. Similar reasoning also applies to the stability of products from vinylic C-H bond activation of L5 and L6. The relative stability of isomeric cycloplatinated compounds 4a and 4b may be due to the different strengths of C-Pt bonds. The steric interaction is the major cause of severe distortion from a planar coordination geometry in the cycloplatinated compounds, which leads to instability of the corresponding cyclometalated products and a higher kinetic barrier for C-H bond activation.

Full text of DOI:10.1021/acs.organomet.5b00326

Article
pubs.acs.org/Organometallics
Computational and Experimental Study on Selective sp2/sp3 or
Vinylic/Aryl Carbon−Hydrogen Bond Activation by Platinum(II):
Geometries and Relative Stability of Isomeric Cycloplatinated
Compounds
Yumin Li,* Jeffrey Carroll, Bradley Simpkins, Deepak Ravindranathan, Christopher M. Boyd,
and Shouquan Huo*
Department of Chemistry, East Carolina University, Greenville, North Carolina 27858, United States
S
* Supporting Information
ABSTRACT: Cyclometalating ligands 6-(1-phenylethyl)-2,2′-bipyr-
idine (L4), 6-(1-phenylvinyl)-2,2′-bipyridine (L5), and 6-(prop-1-
en-2-yl)-2,2′-bipyridine (L6) were synthesized by the Negishi
coupling of 6-bromo-2,2′-bipyridine with the corresponding organo-
zinc reagents. The reaction of L4 with K2PtCl4 produced only the
cycloplatinated compound 4a via sp2 C−H bond activation. The
reactions of L5 and L6 produced exclusively the cycloplatinated
compounds 5b and 6a, respectively, via vinylic C−H bond
activation. DFT calculations were performed on 12 possible
cycloplatination products from the reaction of N-alkyl-N-phenyl-
2,2′-bipyridin-6-amine (alkyl = methyl (L1), ethyl (L2), and
isopropyl (L3)) and L4−L6. The results show that compounds 1b−3b resulting from the sp3 C−H bond activation of L1−
L3 are thermodynamic products, and their relative stability is attributed to the planar geometry that allows for a better
conjugation. Similar reasoning also applies to the stability of products from vinylic C−H bond activation of L5 and L6. The
relative stability of isomeric cycloplatinated compounds 4a and 4b may be due to the different strengths of C−Pt bonds. The
steric interaction is the major cause of severe distortion from a planar coordination geometry in the cycloplatinated compounds,
which leads to instability of the corresponding cyclometalated products and a higher kinetic barrier for C−H bond activation.
Scheme 1. C−H Bond Activation of 6-(N-Alkyl-N-
phenylamino)-2,2′-bipyridine by K2PtCl4
INTRODUCTION
■
Selectivity is a fundamental issue in chemical reactions. It is
common that an organic or organometallic reaction can
produce multiple products through different pathways. The
product distribution is determined by the thermodynamic and
kinetic factors of the reaction that are often influenced by
reaction conditions, and selective formation of the desired
product could thus be achieved by either thermodynamic
control or kinetic control. In a reaction in which two or more
isomeric compounds could be formed through similar and
competitive mechanisms, a selective formation of one isomer is
usually challenging but the most desirable in terms of efficiency
in synthesis and product isolation and purification. In the
recently reported cycloplatination of N-alkyl-N-phenyl-2,2′-
bipyridin-6-amine (alkyl = methyl (L1), ethyl (L2), and
isopropyl (L3)), the selective formation of the product from
either the sp2 C−H or the sp3 C−H activation is controlled by
the solvent used in the reaction (Scheme 1).1 Selective sp2/sp3
C−H bond activation has been reported in other cyclo-
platination reactions,2 and more examples can be found in the
related cyclopalladation reactions;3 however, cases with such a
degree of control in which either isomer can be prepared with
excellent selectivity are rare.1,3a Experimental results suggest
that the product 1b resulting from the sp3 C−H bond
activation of N-methyl-N-phenyl-2,2′-bipyridin-6-amine (L1) in
acetic acid may be more stable, and the reaction is
thermodynamically controlled.1 One difference between 1a
and 1b is that 1a has a fused five−six-membered metallacyle,
while 1b has a fused five−five-membered ring. Although it has
been suggested that a five-membered metal chelation is more
Received: April 17, 2015
© XXXX American Chemical Society
A
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stable than a six-membered chelation,4 and in fact, six-
membered cyclometalated platinum or palladium complexes
are less common, other factors could also contribute to the
relative stability of isomeric sp2 and sp3 C−H bond activation
products a and b.
phenylethyl)picolinonitrile, and the cocyclotrimerization of the
nitrile with acetylene in the presence of (η5-cyclopentadienyl)-
cobalt-1,5-cyclooctadiene.8 We report here a much simpler and
straightforward synthesis of L4. By using Negishi coupling of
(1-phenylethyl)zinc bromide,9 which was generated in situ
from direct insertion of zinc to (1-bromoethyl)benzene,10 with
6-bromo-2,2′-bipyridine, ligand L4 was synthesized in good
yield (Scheme 2). It was found that the reaction of L4 with
K2PtCl4 in either acetic acid or acetonitrile produced 4a
To further understand the selective C−H bond activation, it
is necessary to carry out a systematic study on a series of closely
related reactions. Especially, a combination of complementary
experimental and theoretical studies would allow elucidation of
details of the structure and relative stability of the products that
could be formed in these reactions. Computational chemistry
has played an increasingly important role in studying the
mechanisms of organometallic C−H bond reactions.5 In
particular, density functional theory (DFT) has been frequently
used to investigate the reaction mechanisms of real organo-
metallic transformations including cycloplatination reactions,6
since it allows for accurate calculations of a relatively large
metal complex. Therefore, we decided to use DFT calculations
to examine the relative stability of 12 isomeric cycloplatinated
complexes that could be formed from cycloplatination of L1−
L3 and structurally related ligands L4−L6 (Chart 1) and
1
through sp2 C−H bond activation. The H NMR spectrum of
4a is consistent with that reported previously for the same
compound prepared under different conditions,7,11 showing the
metalation at the phenyl ring. Complete assignment of the
signal was accomplished by the 2D COSY experiments.
Notably, H-6′ appears at 9.70 ppm and H-6″ appears at 8.10
ppm with the Pt−H-6″ coupling constant being 47 Hz. The
reaction in acetonitrile is very sluggish, and no formation of 4b
was detected. These results indicate that 4a could be both the
kinetic and thermodynamic product, which is quite different
from the reaction of L1, where the fused five−five-membered
1b is thought to be the thermodynamic product.1 It should be
noted that the sp3 C−H bond activation of 6-alkyl-2,2′-
bipyridine by platinum was reported to preferentially form a
fused five−five-membered platinacycle.12
Chart 1
The design of ligands L5 and L6 serves to further understand
the structural effect on the selectivity of C−H bond activation.
In L5, both vinylic and aryl C−H bonds are expected to have
similar intrinsic reactivity, but the steric effect and the size of
fused metallacycles may play a role in determining the product
distribution in the cycloplatination reaction. In L6, the methyl
(or allylic) C−H bonds and the vinylic C−H bonds may have
different reactivity, but the products would have the same size
of fused five−five-membered metallacycles. Both ligands L5
and L6 were prepared in high yields via Negishi coupling13 in
the presence of the catalyst Pd(PPh3)4, as shown in Scheme 3.
Interestingly, reactions of L5 and L6 with K2PtCl4 produced
cycloplatinated compounds 5b and 6a, respectively, as the sole
product resulting from the vinylic C−H bond activation,
regardless of the solvent used in the reaction: acetic acid or
acetonitrile. Compounds 5a and 6a were characterized by
compare the results with those obtained experimentally to
elucidate the thermodynamic control of the reaction. The
cycloplatination of L4 has been previously reported,7 but we
will reexamine it under different conditions. Ligands L5 and L6
are newly designed to examine the effect of structural variations
on the control of selectivity in the cycloplatination reactions.
RESULTS AND DISCUSSION
■
1
elemental analysis, mass spectrometry, and H NMR spectros-
Cycloplatination of Ligands L4−L6. Ligand L4 is
structurally related to L1, with the only difference in the
linking atom between the bipyridyl and the phenyl groups. It
would be interesting to see if the solvent-controlled selective
C−H bond activation of L1 is also applicable to L4. The ligand
L4 was synthesized previously by a very sophisticated synthesis
involving oxidation of 2-(1-phenylethyl)pyridine to the
corresponding N-oxide, sequential treatments of the N-oxide
with dimethyl sulfate and potassium cyanide to give 6-(1-
copy. The 13C NMR spectra were not recorded because of poor
solubility of the complexes. The 1H NMR spectra of 5b and 6a
show that the metalation occurs at the vinylic carbon, and the
remaining vinylic hydrogen in 5a and 6a shifts to lower field
upon metalation with a large Pt−H coupling constant,
appearing at 7.95 (2JPt−H = 57 Hz) and 7.54 ppm (2JPt−H
=
61 Hz), respectively. The chemical shifts of 6′ hydrogens in 5a
and 6b are 8.97 and 8.96 ppm, respectively, which is
Scheme 2. Preparation of L4 and Its Cycloplatination Reaction
B
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Scheme 3. Synthesis of Ligands L5 and L6 and Their Cycloplatination Reactions
significantly smaller than that of H-6′ (9.70 ppm) in 4a. Such a
difference in deshielding effect on the H-6′ due to different
sizes of fused platinacycles has been noted before.12 The low
yield of the reaction of L6 in acetic acid is mainly attributed to
the decomposition of platinum complexes since some black
precipitates were formed during the reaction. The decom-
position was not observed in the reaction in acetonitrile. When
the reactions of L5 and L6 were run in AcOD, no deuterium
scrambling was detected at the phenyl group and the methyl
group in the products 5b and 6a. These results suggest that
formation of 5b and 6a might be both kinetically and
thermodynamically controlled.
Chart 3
the other is irreversible (III), the thermodynamic product C
would be the sole product, and the difference in their stability
may have to be determined with a different method if the
equilibrium could not be established between B and C.
The exclusive formation of 5b and 6a can be rationalized by
considering the steric interaction between one of the vinylic H’s
and the 5-H of the pyridine ring in L5 and L6 (Chart 2), which
The cycloplatination of L1 in acetic acid very much
resembles scenario III. Although 1b was suggested to be the
thermodynamic product in the reaction of L1, the formation of
1b seemed to be irreversible, as suggested by a previous
deuterium-labeling experiment, because the refluxing of 1b in
AcOD resulted in D scrambling only at the methylene carbon,
but not at the ortho positions of the phenyl ring.1a On the other
hand, the isomerization of 1a or the cycloplatination of L1 in
AcOD resulted in about 90% D incorporation at both the
methylene and the ortho positions of the benzene ring. Even
with addition of concentrated DCl−D2O, the reflux of 1b in
AcOD for 12 h did not lead to D incorporation to the ortho
positions of the N-phenyl ring (Scheme 4). These D/H
exchange experiments also suggest that in the reaction of L1
with K2PtCl4 in acetic acid the likely reaction pathways involve
the initial coordination of L1 to the platinum salt to generate 7
and 8, cyclometalation of 7 and 8 to produce 1a and 9,
respectively, and a retro “rollover” cyclometalation14 from 9 to
1a and isomerization of 1a to 1b. A “rollover” cyclometalation
from 7 to 9 may not be a favorable pathway under the
conditions, as the isomerization of 1a to 1b in AcOD, which
proceeds likely through deuterolysis of 1a to give 7-d
(deuterated at the metalated carbon of the phenyl group) as
the intermediate, did not lead to a measurable D/H exchange at
the 3-position of the bipyridine ring. If the equilibrium between
the two isomers 1a and 1b could not be established with an
experiment, theoretical calculations would be the alternative
Chart 2
occurs when L5 and L6 have to adopt a proper conformation
for the aryl C−H bond activation of L5 and methyl (allylic) C−
H bond activation of L6. Additional steric interaction exists
between the other vinylic H and one of the ortho H’s of the
phenyl ring in L5. It can be expected that the stability of 5a and
6b would also suffer from such steric hindrance.
In general, for competing sp2/sp3 or vinylic/aryl C−H bond
activations of compound A producing cycloplatinated com-
pounds B and C, there are three possible scenarios, I, II, and III,
as shown in Chart 3. If both reactions are irreversible, the
product formation will be under kinetic control (I). On the
other hand, if both reactions are reversible, the product
formation will be thermodynamically controlled (II), and the
ratio of the products is a reflection of their relative
thermodynamic stability. If one reaction is reversible, while
C
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Scheme 4. D/H Exchange and “Rollover” Cyclometalation in the Reaction of L1 with K2PtCl4 in AcOH(D)
Figure 1. Optimized geometries of 1a−3a and 1b−3b.
tool to study the relative stability of the compounds. Moreover,
compounds 4b, 5a, and 6b were not formed; thus no
experimental information on the structure and stability of
these compounds was available. Therefore, we decided to
conduct a systematic study on 12 possible cycloplatinated
compounds from the cycloplatination of L1−L6 using DFT
calculations to elucidate the molecular structures and estimate
the relative stability of the isomeric complexes.
X-ray structures of 3a and 3b have been reported,1a
a
systematic study on the geometry and stability of this series
of platinum complexes is necessary to understand the selectivity
of the reaction. In particular, a systematic change in the N-alkyl
groups from methyl to isopropyl provides an excellent model to
assess the steric effect of the alkyl group on the geometry and
stability of these cyclometalated platinum compounds. Since
the structures of 3a and 3b have been determined by X-ray
crystallography, their structural parameters were used as the
starting point of the optimization. Figure 1 shows optimized
DFT Calculations on Compounds 1a−3a and 1b−3b.
Geometric Optimization of 1a−3a and 1b−3b. Although the
D
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Table 1. Optimized Bond Lengths (Å), Bond Angles (deg), and Torsional Angles (deg) of 1a−3a and 1b−3b
1a
2a
3a
gas state
in MeCN
in AcOH
gas state
in MeCN
in AcOH
gas state
in MeCN
in AcOH
X-ray1a
Pt−Cl
Pt−C(1)
Pt−N2
Pt−N3
N1−C2
N1−C3
N1−C4
C(1)−Pt−N3
Cl−Pt−N3
Cl−Pt−C(1)
C(1)−Pt−N2
N2−Pt−N3
N2−Pt−Cl
C2−N1−C3
C3−N1−C4
C2−N1−C4
2.332
1.963
2.038
2.174
1.438
1.458
1.367
168.3
92.6
2.372
1.970
2.026
2.176
1.429
1.462
1.374
168.3
93.0
2.362
1.967
2.030
2.178
1.431
1.461
1.372
167.9
93.0
2.330
1.962
2.041
2.178
1.438
1.470
1.368
167.0
93.2
2.371
1.968
2.026
2.179
1.431
1.472
1.375
167.2
93.6
2.362
1.966
2.030
2.179
1.433
1.472
1.374
167.1
93.5
2.328
1.959
2.045
2.182
1.438
1.489
1.369
165.8
93.2
2.373
1.965
2.029
2.179
1.431
1.491
1.374
166.8
93.2
2.363
1.964
2.032
2.180
1.433
1.490
1.373
166.7
93.2
2.310
1.997
2.000
2.091
1.431
1.503
1.375
167.3
93.1
95.4
95.1
95.2
95.4
95.1
95.1
95.6
95.3
95.4
94.7
93.5
93.0
93.1
93.1
92.7
92.8
93.1
92.7
92.8
91.7
79.1
79.4
79.3
78.9
79.2
79.1
78.9
79.5
79.4
80.7
170.7
116.9
116.7
125.5
171.4
117.2
116.6
124.6
1b
171.1
117.1
116.7
125.8
171.2
117.9
117.8
123.8
172.0
118.3
117.6
122.9
2b
171.8
118.2
117.6
123.1
170.7
118.9
116.5
123.7
171.5
120.0
116.4
123.6
171.3
119.8
116.4
123.7
173.5
120.9
115.6
123.0
3b
gas state
in MeCN
in AcOH
gas state
in MeCN
in AcOH
gas state
in MeCN
in AcOH
X-ray1a
Pt−Cl
Pt−C(1)
Pt−N2
Pt−N3
N1−C(1)
N1−C2
N1−C3
C(1)−Pt−N3
Cl−Pt−N3
Cl−Pt−C(1)
C(1)−Pt−N2
N2−Pt−N3
N2−Pt−Cl
C2−N1−C3
C2−N1−C(1)
C(1)−N1−C3
2.332
1.983
1.963
2.212
1.502
1.347
1.426
161.9
99.1
2.383
1.990
1.957
2.216
1.493
1.348
1.429
161.8
100.3
97.9
2.372
1.989
1.958
2.216
1.495
1.347
1.428
161.8
100.1
98.1
2.336
1.990
1.963
2.216
1.518
1.350
1.425
162.5
99.1
2.387
1.996
1.956
2.222
1.509
1.352
1.427
162.4
100.0
97.7
2.376
1.995
1.957
2.221
1.511
1.351
1.427
162.4
99.7
2.341
2.002
1.961
2.221
1.535
1.348
1.430
162.9
99.3
2.390
2.009
1.956
2.230
1.525
1.349
1.432
162.7
99.8
2.380
2.008
1.957
2.223
1.528
1.349
1.431
162.8
99.6
2.303
2.026
1.944
2.115
1.543
1.344
1.432
163.1
99.9
99.0
98.5
97.9
97.8
97.5
97.6
96.5
84.4
84.0
84.1
84.9
84.5
84.6
85.5
85.1
85.2
84.4
77.5
77.8
77.7
77.6
77.9
77.8
77.4
77.6
77.6
79.3
176.6
119.3
121.0
119. 7
178.1
119.2
121.4
119.4
177.8
119.2
121.3
119.4
176.6
119.3
121.0
119.8
177.8
119.1
121.3
119.5
177.5
119.2
121.2
119.6
176.7
120.6
119.1
120.2
177.9
120.7
119.3
120.0
177.2
120.7
119.2
120.0
178.3
119.6
118.9
121.0
geometries of 1a−3a and 1b−3b in their gas state. The major
bond parameters around the coordination center and the amino
nitrogen center for the optimized geometries in both the gas
state and solvated states are listed in Table 1. The bond
parameters from the X-ray crystal structures of 3a and 3b are
also listed for comparison. Major bond lengths and angles for
optimized geometries of 3a and 3b compare favorably with
those determined by X-ray crystallography. It should be noted
that the optimized geometry is for the molecule in its gas phase
and solvated state, while the X-ray crystallography determines
the solid-state structure. Therefore, small variations on the
geometry in different states are expected. Nonetheless, the
results suggest that the DFT method used here is very reliable
in predicting the molecular geometry of the platinum
complexes. The geometries of the compounds at their solvated
states display a longer Pt−Cl bond distance by 0.04 and 0.05 Å
in MeCN and 0.03 and 0.04 Å in AcOH compared with those
in their gas states for 1a−3a and 1b−3b, respectively. This can
be reasoned by the polarity of the Pt−Cl bond, because
solvation assists the dissociation of a polar bond by stabilizing
the charged species. Changes to the other bonds vary and are
insignificant.
Complexes of platinum(II) with four ligands prefer a square
planar geometry. When the four ligands are different, the square
geometry may be distorted as the bond lengths between the
platinum and the donor atoms of the ligands and bite angles of
the ligands can be different, but a planar geometry is generally
retained to maintain the efficient bonding between the metal
center and the donor ligands, particularly those with extended
conjugation. However, other factors such as torsional and angle
strain may force the platinum coordination out of an ideal
plane. Comparison between isomeric compounds a and b
reveals that the coordination in complexes 1a−3a is
significantly deviated from a planar geometry, while the
coordination geometry in 1b−3b is nearly a perfect plane.
Figure 2 displays the views of compounds 1a−3a and 1b−3b
by projecting through the platinum coordination plane.
In 1b−3b, the chlorine, the platinum, the carbon donor C1,
the bipyridyl ring, and the linker amino nitrogen N1 are
coplanar, as shown in Figure 2. The mean planes composed of
the 12 atoms of the bipyridyl ring were calculated for 1a−3a
and 1b−3b, and the deviations of the atoms from the plane are
listed in Table 2. Also listed in Table 2 are the deviations of Pt,
Cl, the amino nitrogen (N(1)), and the metalated carbon
(C(1)) from the bipyridyl plane. In 3b and 1b, the
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spacious orientation of the methyl group repelling the phenyl
ring and making it slightly twisted from a perpendicular
orientation. The steric repulsion between the phenyl ring and
the methyl group(s) can be seen clearly from small but distinct
changes in angles C1−N−C3, C2−N−C3, and C1−N−C2
from 1b to 2b and to 3b. As the number of methyl groups
increases from zero in 1b to two in 3b, the steric hindrance
increases. As a consequence, C1−N−C3 angles increase and
C2−N−C3 angles decrease, while C1−N−C2 angles stay
constant. In other words, the phenyl group is pushed away from
the methyl group(s). The perpendicular orientation adopted by
the phenyl ring is apparently to minimize the steric strain.
Another consequence of the increasing steric interaction of the
methyl group(s) is the stretching of the sp3 C−N bond, which
elongates gradually from 1.502 Å in 1b to 1.518 and 1.535 Å in
2b and 3b, respectively. For the same reason, the C−Pt bond
increases from 1.983 Å in 1b to 2.002 Å in 3b. The amino
nitrogen adopts a perfect trigonal planar geometry, and the
trigonal plane is coplanar with the bipyridyl ring with dihedral
angles of 0.02°, 1.74°, and 0.07° for 1b−3b, respectively.
The geometry of 1a to 3a is far from coplanar, showing
significant bending of the phenyl ring relative to the platinum
coordination plane and even twisting and bending of the two
pyridyl rings (Figure 2). The dihedral angle of two pyridyl rings
is 12.78°, 12.53°, and 17.87° for 1a−3a, respectively. The
dihedral angle between the phenyl ring and the bipyridyl ring is
31.19°, 35.86°, and 36.45° in 1a, 2a, and 3a, respectively. The
platinum and chlorine are approximately coplanar with the
bipyridyl ring with a <0.3 Å deviation; however, the metalated
carbons are severely deviated from the pyridyl plane, with a
distance of 0.871, 0.960, and 0.968 Å in 1a−3a, respectively.
The amino nitrogen (N1) adopts a slightly distorted trigonal
planar geometry. The distance between the nitrogen and the
trigonal plane composed of the three carbon atoms that bond
to the nitrogen is 0.077, 0.056, and 0.078 Å for 1a, 2a, and 3a,
respectively. The dihedral angles between the trigonal plane
and the bipyridyl ring are 32.51°, 38.85°, and 44.35° for 1a−3a,
respectively. The amino nitrogen deviates from the pyridyl
plane by 0.027, 0.022, and 0.177 Å in 1a, 2a, and 3a,
respectively.
Figure 2. View of molecules by projecting through the bipyridyl ring
in 1b−3b and 1a−3a.
Table 2. Deviations (Å) of Platinum, Chlorine, Amino
Nitrogen N(1), Metalated Carbon C(1), Carbon, and
Nitrogen Atoms in the Bipyridyl Ring from the Mean Plane
of the Bipyridyl Ring
atom
1a
1b
2a
2b
3a
3b
a
Pt
0.273
0.161
0.871
0.0027
0.058
0.090
0.142
0.071
0.090
0.071
0.150
0.061
0.100
0.146
0.182
0.061
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.001
0.001
0.000
0.000
0.299
0.229
0.960
0.022
0.053
0.081
0.139
0.077
0.087
0.066
0.147
0.063
0.093
0.143
0.183
0.066
0.021
0.067
0.024
0.022
0.016
0.013
0.019
0.004
0.008
0.004
0.014
0.009
0.007
0.017
0.018
0.011
0.251
0.014
0.968
0.177
0.119
0.106
0.210
0.109
0.107
0.080
0.207
0.109
0.114
0.208
0.238
0.112
0.001
0.001
0.003
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
a
Cl
a
C(1)
a
N1
b
Cpy
Cpy
Cpy
Cpy
Cpy
Cpy
Cpy
Cpy
Cpy
Cpy
N2
N3
The distorted geometry of 1a−3a contrasts sharply to 1b−
3b. One factor that should be considered is the different fused
metallacycles. The former has a five−six fused metallacycle,
while the latter has a five−five fused ring. The different size of
the fused rings and nature of the C−Pt and N−Pt bonds may
induce different angle strain. The steric effect incurred by the
N-alkyl groups is perhaps the major contribution to the
distorted geometry of 1a−3a, as other similar molecules with a
five−six fused metallacycle and a N-phenyl group rather than a
N-alkyl group show a geometry much closer to a plane15
because the N-phenyl ring can take a perpendicular orientation
to avoid the steric interaction as mentioned above.
a
Not included in calculating the mean coordination planes in 1a−3a.
Cpy denotes carbon atoms in the bipyridyl ring.
b
coordination plane is perfect. The largest deviation was found
to be only 0.003 Å in 3b. The coordination geometry in 2b is
slightly deviated from a perfect plane, with the largest deviation
of 0.043 Å. The phenyl ring in 1b, 2b, and 3b is nearly
perpendicular to the coordination plane, with dihedral angles of
89.95°, 65.71°, and 89.86°, respectively. The relatively small
dihedral angle in 2b may be attributed to the unsymmetrical
Relative Stability of the Isomeric Platinum Compounds.
The calculated energies of the optimized geometries are
Table 3. Solvation Energies (ΔEsol in kcal/mol) and Energy Differences (ΔE) between Isomeric sp2/sp3 C−H Bond Activation
Products 1a,b−3a,b
ΔEsol (1a)
ΔEsol (1b)
ΔE (1b−1a)
ΔEsol (2a)
ΔEsol (2b)
ΔE (2b−2a)
ΔEsol (3a)
ΔEsol (3b)
ΔE (3b−3a)
gas state
In MeCN
In AcOH
−2.19
−0.78
−1.31
−5.65
−3.71
−4.33
−8.26
−6.08
−6.75
−16.92
−13.17
−15.51
−12.29
−16.79
−12.94
−14.85
−11.62
−16.36
−12.61
−14.18
−11.10
F
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Figure 3. Optimized geometries of 4a−6a and 4b−6b.
Table 4. Optimized Bond Lengths (Å), Bond Angles (deg), and Torsional Angles (deg) of 4a−6a and 4b−6b
4a
5a
6a
gas state
in MeCN
in AcOH
gas state
in MeCN
in AcOH
gas state
in MeCN
in AcOH
Pt−Cl
Pt−C(1)
Pt−N1
Pt−N2
C(1)−Pt−N2
Cl−Pt−N2
Cl−Pt−C(1)
C(1)−Pt−N1
N1−Pt−N2
N1−Pt−Cl
2.321
1.976
2.045
2.196
165.3
94.1
2.361
1.979
2.035
2.196
164.8
94.8
2.351
1.978
2.038
2.197
164.9
94.8
2.324
1.974
2.048
2.190
168.3
92.9
2.361
1.979
2.039
2.190
168.0
93.5
2.353
1.978
2.041
2.190
168.0
93.4
2.318
1.941
1.971
2.226
159.4
99.8
2.369
1.951
1.968
2.234
159.5
101.0
99.5
2.358
1.949
1.968
2.231
159.5
100.7
99. 9
81.7
95.1
94.6
94.6
94.8
94.3
94.5
100.8
81.8
93.3
92.8
92.9
94.5
94.1
94.2
81.7
77.8
78.1
78.0
78.2
78.5
78.4
77.6
77.8
77.7
171.6
172.6
4b
172.5
170.5
171.3
5b
171.1
177.4
178.8
6b
178.4
gas state
in MeCN
in AcOH
gas state
in MeCN
in AcOH
gas state
in MeCN
in AcOH
Pt−Cl
Pt−C(1)
Pt−N1
Pt−N2
C1−C2
C2−C3
C2−C4
C(1)−Pt−N2
Cl−Pt−N2
Cl−Pt−C(1)
C(1)−Pt−N1
N1−Pt−N2
N1−Pt−Cl
C1−C2−C3
C3−C2−C4
C1−C2−C4
2.322
1.995
1.974
2.227
1.562
1.513
1.514
161.8
99.1
2.376
2.000
1.971
2.227
1.561
1.514
1.513
161.9
100.0
97.7
2.364
2.000
1.972
2.229
1.561
1.513
1.513
161.7
99.8
2.316
1.935
1.973
2.221
1.370
1.484
1.467
159.7
99.6
2.368
1.943
1.970
2.224
1.367
1.484
1.473
159.8
101.0
99.2
2.357
1.941
1.970
2.223
1.367
1.484
1.472
159.8
100.7
99.6
2.324
2.000
1.972
2.216
1.526
1.340
1.483
162.4
98.9
2.376
2.006
1.970
2.221
1.525
1.340
1.481
162.3
99.9
2.365
2.004
1.970
2.220
1.526
1.340
1.482
162.3
99.8
98.6
98.0
100.7
81.8
98.4
97.6
97.6
84.6
84.3
84.3
81.7
81.7
84.8
84.5
84.6
77.6
78.0
77.9
77.9
78.1
78.1
77.9
78.1
78.1
176.8
114.1
112.9
110.6
177.9
113.2
113.4
110.3
177.6
113.6
113.3
110.3
177.4
123.9
121.4
114.7
178.7
123.7
121.7
114.6
178.4
123.8
121.6
114.7
176. 8
124.6
120.9
114.5
177.8
124.5
120.9
114.5
177.8
124.5
120.8
114.6
corrected with zero-point energies. The solvation energy and
the relative stability of isomers are summarized in Table 3. The
solvation energy is expressed as the energy difference between
those in the gas and solvated states, ΔEsol = Esol − Egas, with a
negative value indicating a stabilization effect, where Esol is the
calculated energy of the molecule in its solvated state and Egas is
the energy of the molecule in its gas state. The relative stability
of isomers is expressed as the energy difference between the
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two isomers b and a, ΔE = Eb − Ea. The results can be
summarized as follows. First, in both the gas phase and the
solvated state, all the sp3 C−H activation products 1b, 2b, and
3b are predicted to be more stable than their isomeric sp2 C−H
activation products 1a, 2a, and 3a, respectively, and the energy
difference between the two isomers increases with the
increasing size of the N-alkyl groups from methyl (1a and
1b) to ethyl (2a and 2b) to isopropyl (3a and 3b). These
results are consistent with the experimental finding that 1b−3b
are the thermodynamic products and 1a−3a are kinetic
products.1 Second, solvents significantly stabilize the cyclo-
metalated complexes (by 11−17 kcal/mol), and the stabiliza-
tion slightly decreases from 1a and 1b to 3a and 3b.
Acetonitrile stabilizes the complexes by about 3−4 kcal/mol
more than acetic acid does. Finally, the solvent stabilization on
1a, 2a, and 3a is slightly higher than that on 1b, 2b, and 3b, by
about 1−2 kcal/mol.
The greater stability of 1b−3b can be reasoned by their
planar coordination geometry, which maximizes the electron
delocalization stabilization through conjugation between the
bipyridyl ring, the amino, and the platinum center.
Figure 4. Molecular view of 4a−6a and 4b−6b by projecting through
the bipyridyl ring.
DFT Calculations on 4a−6a and 4b−6b. Geometric
Optimization of 4a−6a and 4b−6b. The optimized geo-
metries are shown in Figure 3. The structural parameters
around the coordination sphere of optimized geometries
including both the gas state and the solvated states are listed
in Table 4. It can be seen from the table that the geometries of
the compounds in their solvated states display a longer Pt−Cl
bond distance compared with those in their gas states by 0.05
and 0.04 Å in MeCN and AcOH, respectively. The changes to
other bonds are not significant. The mean plane of the bipyridyl
ring is calculated, and the deviations of the atoms from the
plane are listed in Table 5. Also listed are the deviations of
other atoms in the coordination sphere. Figure 4 shows the
view of molecular structures of 4a−6a and 4b−6b by projecting
through the plane of the bipyridyl ring. Compound 4a has a
fused five−six-membered metallacyle and displays a signifi-
cantly distorted geometry from a planar coordination. Even the
bipyridyl ring is slightly twisted and bent, with the dihedral
angle between the two pyridyl rings being 8.52°. The platinum,
chlorine, and the carbon donor C(1) are deviated from the
bipyridyl ring by 0.540, 0.782, and 1.275 Å, respectively. The
dihedral angle between the phenyl ring and the bipyridyl ring is
49.42°. The six-membered metallacyle adopts a boat-like
conformation. In 4b, the bipyridyl ring is planar, and Pt, Cl,
carbon donor C(1), and the linking carbon atom C(2) deviate
from the plane by 0.138, 0.281, 0.365, and 0.147 Å, respectively.
The platinum forms a stronger C−Pt bond in 4a than that in
4b, presumably due to the nature of the carbon donors: an sp2
carbon normally forms a stronger bond than an sp3 carbon.
The coordination geometry in compound 5a is also severely
distorted from a plane. It is interesting to note that the
geometries of 4a and 5a are very similar to each other (Figures
3 and 4), although the linking atom between the bipyridyl and
the phenyl rings is an sp3 carbon in 4a but an sp2 carbon in 5a.
The bipyridyl ring is also twisted and bent (dihedral angle of
8.92°). The platinum, chlorine, and the carbon donor C(1)
deviate from the bipyridyl ring by 0.400, 0.504, and 1.052 Å,
respectively. The dihedral angle between the phenyl and the
bipyridyl rings is 37.25°. The vinyl group is bent with respect to
the bipyridyl plane with the terminal vinyl carbon deviated by
1.090 Å. The vinyl group maintains a planar geometry, but
forms a dihedral angle of 44.46° and 49.27° with the phenyl
ring and the bipyridyl ring, respectively. Such significant
deviation from the planar geometry is likely the result of
balance between steric interaction (Chart 3) and electron
delocalization. On the contrary, compound 5b, with metalation
at the vinyl carbon, displays nearly perfect planar coordination
geometry. The chlorine shows the largest deviation from the
bipyridyl plane, but by only 0.1 Å. The phenyl ring adopts a
twisted orientation, forming a dihedral angle of 49.91° with the
coordination plane to minimize the steric interaction. The C−
Pt bond in compound 5b is also stronger than that of 5a (1.935
vs 1.974 Å), even though both are sp2 C−Pt bonds.
Table 5. Deviations (Å) of Platinum, Chlorine, Carbon, and
Nitrogen Atoms in the Bipyridyl Ring from the Mean Plane
of the Bipyridyl Ring
atom
4a
4b
5a
5b
6a
6b
a
Pt
0.504
0.782
1.275
0.902
0.277
0.051
0.120
0.050
0.024
0.101
0.072
0.053
0.033
0.093
0.055
0.018
0.134
0.138
0.281
0.365
0.167
0.410
0.023
0.013
0.006
0.008
0.013
0.007
0.005
0.013
0.008
0.004
0.015
0.008
0.400
0.504
1.052
0.144
1.090
0.014
0.101
0.079
0.011
0.090
0.068
0.085
0.002
0.091
0.085
0.145
0.009
0.026
0.104
0.013
0.020
0.048
0.023
0.011
0.024
0.009
0.007
0.003
0.019
0.015
0.006
0.023
0.022
0.017
0.000
0.000
0.001
0.001
0.002
0.001
0.000
0.001
0.000
0.000
0.000
0.001
0.000
0.001
0.000
0.000
0.000
0.095
0.176
0.266
0.084
0.507
0.015
0.007
0.001
0.001
0.011
0.006
0.003
0.005
0.008
0.004
0.018
0.003
a
Cl
a
C(1)
a
C2
a
C3
b
Cpy
Cpy
Cpy
Cpy
Cpy
Cpy
Cpy
Cpy
Cpy
Cpy
N1
N2
Compound 6a has perfect planar coordination geometry, but
its isomer 6b shows distorted geometry because of steric
interaction between the linking vinyl group and the bipyridyl
group. The platinum in 6b is only slightly deviated from the
a
b
Not included in calculating the mean coordination planes. Cpy
denotes carbon atoms in the bipyridyl ring.
H
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Table 6. Solvation Energies (ΔEsol in kcal/mol) and Energy Differences (ΔE) between Isomeric C−H Bond Activation Products
4a,b−6a,b
ΔEsol (4a)
ΔEsol (4b)
ΔE (4b−4a)
ΔEsol (5a)
ΔEsol (5b)
ΔE (5b−5a)
ΔEsol (6a)
ΔEsol (6b)
ΔE (6b−6a)
gas state
in MeCN
in AcOH
2.86
2.83
2.70
−3.56
−2.54
−2.92
6.98
6.07
6.22
−16.89
−13.11
−16.93
−13.28
−16.81
−13.04
−15.79
−12.40
−15.20
−11.89
−16.11
−12.66
bipyridyl ring, by 0.095 Å, but the chlorine and the metalated
carbon C(1) show a 0.176 and 0.266 Å distance to the plane,
respectively. The planar vinyl group is bent from the bipyridyl
plane with the terminal vinyl carbon distanced by 0.507 Å and
forms a dihedral angle of 18.59° with the bipyridyl ring. The
C−Pt bond in 6a (1.941 Å) is much shorter than that in 6b
(2.000 Å), indicative of stronger bonding in 6a.
Relative Stability of Isomeric Cycloplatinated Compounds.
The energies of the optimized geometries were corrected with
zero-point energies. The solvent energies and the energy
differences between isomeric compounds are summarized in
Table 6. In both the gas phase and solvated state, compounds
4a, 5b, and 6a are predicted to be more stable than their
isomers 4b, 5a, and 6b, respectively, which are consistent with
the experimental finding that 4a, 5b, and 6a are formed
exclusively as likely thermodynamic products. Solvents
significantly stabilize the cyclometalated complexes (by 11−
17 kcal/mol), and acetonitrile stabilizes the complexes by about
3−4 kcal/mol more than acetic acid does.
transition states and intermediates in the reaction, and the
results will be reported in due course.
EXPERIMENTAL SECTION
■
Synthesis. All reactions involving moisture- and/or oxygen-
sensitive organometallic complexes were carried out under a nitrogen
or argon atmosphere and anhydrous conditions. Tetrahydrofuran
(THF) and diethyl ether were distilled from sodium and
benzophenone under nitrogen before use. All other anhydrous
solvents were purchased from Aldrich Chemical Co. and were used
as received. 6-Bromo-2,2′-bipyridine17 was prepared according to the
literature procedure. All other reagents were purchased from chemical
companies and were used as received. NMR spectra were measured on
a Bruker 400 or a Varian 500 spectrometer. Spectra were taken in
1
CDCl3 or CD2Cl2 using tetramethylsilane as standard for H NMR
chemical shifts and the solvent peak (CDCl3, 77.0 ppm; CD2Cl2, 53.8
ppm) as standard for 13C NMR chemical shifts. Coupling constants (J)
are reported in Hz. The 2D COSY experiments were performed using
standard pulse sequences. Elemental analyses were performed at
Atlantic Microlab, Inc., Norcross, GA, USA.
Preparation of L4. A 50 mL three-neck round-bottom flask was
charged with LiCl (0.42 g, 10 mmol) and Zn dust (0.65 g, 10 mmol),
dried with a heat gun under vacuum, and backfilled three times with
argon. Tetrahydrofuran (3 mL) and 1,2-dibromoethane (22 μL, 0.25
mmol) were added, and the mixture was heated at 60 °C for 20 min.
After cooling to room temperature, TMSCl (6.3 μL, 0.05 mmol) and
iodine (3.2 mg, 0.025 mmol) in a THF solution were added. This
mixture was heated at 60 °C for 20 min and then cooled to room
temperature, followed by addition of 1-bromoethylbenzene (0.68 mL,
5 mmol). The mixture was stirred at 50 °C for 16 h. In a separate
argon-flushed 50 mL three-neck round-bottom flask, 6-bromo-2,2′-
bipyridine (0.47 g, 2 mmol), PdCl2(dppf) (82 mg, 0.1 mmol), and
THF (2 mL) were added. The supernatant of the Zn reagent mixture
was added dropwise to the reaction flask, and the reaction mixture was
heated at 50 °C for 3 h. After quenching with 20 mL of water, EDTA
(5.8 g, 20 mmol), and Na2CO3 (4.2 g, 40 mmol), the organic products
were extracted with 3 × 75 mL portions of ethyl acetate and the
combined organic layers were washed with brine and dried over
MgSO4. Solvent was removed by rotary evaporation, and the crude
product was purified by column chromatography on silica gel with
hexane and ethyl acetate (v/v 5/1): yellow-brown oil, 0.34 g, 65%. 1H
NMR (400 MHz, CDCl3): δ 8.66 (d, J = 4.8, 1 H), 8.53 (d, J = 8.0, 1
H), 8.21 (d, J = 7.9, 1 H), 7.82 (td, J = 7.8, 1.8, 1 H), 7.68 (t, J = 7.8, 1
H), 7.37 (m, 2 H), 7.30 (m, 3 H), 7.20 (t, J = 7.4, 1 H), 7.10 (d, J =
7.7, 1 H), 4.35 (q, J = 7.3, 1 H), 1.78 (d, J = 7.2, 3 H).
Preparation of L5. A 50 mL three-neck round-bottom flask under
argon was cooled to −78 °C and charged with diethyl ether (3 mL)
and α-bromostyrene (0.25 mL, 2 mmol). The resulting brown mixture
was stirred for 10 min, followed by dropwise addition of t-BuLi (1.7 M
solution in pentane, 2.35 mL, 4 mmol). ZnCl2 (1.0 M solution in
ether, 1 mL, 1 mmol) was added dropwise, and the reaction mixture
was stirred for 30 min before warming to room temperature. Next, 6-
bromo-2,2′-bipyridine (235.1 mg, 1 mmol), Pd(PPh3)4 (57.8 mg, 0.05
mmol), and THF (8 mL) were added, and the mixture was heated at
reflux for 3 h. After quenching with 20 mL of water, EDTA (292 mg, 1
mmol), and Na2CO3 (106 mg, 1 mmol), the organic products were
extracted with three 30 mL portions of ethyl acetate and the combined
organic layers were washed with brine and dried over MgSO4. Solvent
was removed by rotary vapor, and the crude product was purified by
column chromatography on silica gel with hexane and ethyl acetate (v/
v 5/1): yellow-brown solid, 0.22 g, 86%. 1H NMR (400 MHz,
The greater stability of 5b and 6a can be attributed to the
planar geometry of the coordination compounds, which
maximizes the bonding and electron delocalization, particularly
the conjugation between the vinyl group and the bipyridyl ring.
It is also conceivable that the five-membered platinacycle in 5b
and 6a may possess aromatic character.16 The larger difference
between 6a and 6b may be attributed to the stronger vinyl C−
Pt bonding in 6a that adds additional stabilization. On the
other hand, both 4a and 4b have a twisted geometry, but 4a is
calculated to be more stable than 4b by 2.86 kcal/mol in the gas
state. This is at least partially due to the stronger sp2 C−Pt
bonding in 4a than the sp3 C−Pt in 4b.
SUMMARY
■
In the cycloplatintion of 6-substituted 2,2′-bipyridines where
competing sp2/sp3 or vinyl/aryl C−H bond activation is
involved, the outcome of the reaction varies depending on the
structure of the 6-substituents. In the reaction of N-alkyl-N-
phenyl-2,2′-bipyridin-6-amine (alkyl = methyl (L1), ethyl (L2),
and isopropyl (L3)), the selectivity of competing sp2/sp3 C−H
bond activation can be controlled by using different solvents,
resulting in either thermodynamic or kinetic control of the
reaction. DFT calculations predict that the products 1b−3b via
the sp3 C−H activation are more stable than their
corresponding isomers 1a−3a via the sp2 C−H activation,
which confirms the thermodynamic control of the reaction in
acetic acid. On the other hand, the reactions of 6-(1-
phenylethyl)-2,2′-bipyridine (L4), 6-(1-phenylvinyl)-2,2′-bipyr-
idine (L5), and 6-(prop-1-en-2-yl)-2,2′-bipyridine (L6) pro-
duce exclusively 4a, 5b, and 6a, respectively, via sp2 or vinyl C−
H activation. DFT calculations indicate that these compounds
are more stable than their isomers via the other competing C−
H activation. The formation of the sole product is most likely
both kinetically and thermodynamically controlled. We are
currently performing a DFT calculation to simulate the reaction
pathways of cycloplatination of L1 and elucidating the
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CDCl3): δ 8.68 (d, J = 5.2, 1 H), 8.47 (d, J = 8.0, 1 H), 8.35 (d, J = 7.9,
1 H), 7.80 (td, J = 7.7, 1.8, 1 H), 7.76 (t, J = 7.8, 1 H), 7.39 (m, 5 H),
7.30 (m, 1 H), 7.26 (d, J = 7.8, 1 H), 6.25 (d, J = 1.7, 1 H), 5.64 (d, J =
1.7, 1 H). 13C NMR (75 MHz, CDCl3): δ 157.3, 156.3, 155.5, 149.0 (2
C), 140.5, 137.2, 136.9, 128.6 (2 C), 128.2 (2 C), 127.7, 123.7, 122.7,
121.3, 119.7, 117.9. HRMS (ESI-QToF): calcd for C18H15N2 (M +
H)+ 259.12, found 259.03. Anal. Calcd for C18H14N2: C, 83.69; H,
5.46; N, 10.84. Found: C, 83.42; H, 5.66; N, 10.79.
chromatography on silica gel with dichloromethane and ethyl acetate
(v/v 1/1): orange solid, 94.4 mg, 79%.
Reaction of L6 with K2PtCl4 in Acetic Acid: Preparation of 6a. To
a 50 mL round-bottom flask with condenser were added L6 (49 mg,
0.25 mmol), K2PtCl4 (104 mg, 0.25 mmol), and glacial acetic acid (12
mL). The mixture was heated at reflux for 2 h, resulting in a dark
orange mixture with some black precipitate. The solvent was removed
by rotary evaporator, and the crude product was purified by column
chromatography on silica gel with dichloromethane: gold solid, 40.2
Preparation of L6. A 100 mL three-neck round-bottom flask under
argon was cooled to 0 °C and charged with isopropenyl magnesium
bromide (0.5 M solution in THF, 12 mL, 6 mmol). Zinc chloride (1.0
M solution in ether, 6 mL, 6 mmol) was added dropwise, and the
reaction mixture was stirred 15 min before warming to room
temperature. Next, 6-bromo-2,2′-bipyridine (470 mg, 2 mmol) and
Pd(PPh3)4 (116 mg, 0.1 mmol) were added, and the resulting cloudy,
green mixture was heated at reflux for 19 h. After quenching with 20
mL of water, EDTA (3.4 g, 12 mmol), and Na2CO3 (5.1 g, 48 mmol),
the organic products were extracted with three 30 mL portions of ethyl
acetate and the combined organic layers were washed with brine and
dried over MgSO4. Solvent was removed by rotary vapor, and the
crude product was purified by column chromatography on silica gel
with hexane and ethyl acetate (v/v 5/1): yellow-brown oil, 0.32 g,
82%. 1H NMR (400 MHz, CDCl3): δ 8.66 (d, J = 4.8, 1 H), 8.53 (d, J
= 8.0, 1 H), 8.31 (d, J = 7.8, 1 H), 7.81 (td, J = 7.8, 1.8, 1 H), 7.78 (t, J
= 7.8, 1 H), 7.51 (d, J = 4.8, 1 H), 7.29 (m, 1 H), 6.00 (s, 1 H), 5.35 (s,
1 H), 2.30 (s, 3 H). 13C NMR (75 MHz, CDCl3): δ 157.4, 156.5,
154.9, 149.0, 143.4, 137.2, 136.8, 123.6, 121.2, 119.6, 119.2, 115.5,
20.5. HRMS (ESI-QToF): calcd for C13H13N2 (M + H)+ 197.11,
found 197.09. Anal. Calcd for C13H12N2: C, 79.56; H, 6.16; N, 14.27.
Found: C, 79.04; H, 6.42; N, 14.23.
1
3
mg, 38%. H NMR (400 MHz, CD2Cl2): δ 8.96 (d, J = 5.8, JPt−H
=
12.8, H-6′), 8.12 (td, J = 7.9, 1.6, H-4′), 7.94 (d, J = 7.9, H-3′), 7.80 (t,
J = 7.9, H-4), 7.72 (dd, J = 7.7, 1.2, H-5′), 7.54 (s, 2JPt−H = 60.6, vinyl-
H), 7.50 (d, J = 8.0, H-3), 6.97 (d, J = 8.0, 4JPt−H = 11.4, H-6), 2.09 (s,
CH3). HRMS (ESI-QToF): calcd for acetonitrile complex,
C15H14N3Pt (M)+ 431.1, found 431.1. Anal. Calcd for C13H11ClN2Pt:
C, 36.67; H, 2.60; N, 6.58. Found: C, 36.14; H, 2.61; N, 6.44.
Reaction of L6 with K2PtCl4 in Acetonitrile. To a 50 mL round-
bottom flask with condenser were added L6 (49 mg, 0.25 mmol),
K2PtCl4 (104 mg, 0.25 mmol), and acetonitrile (12 mL). The mixture
was heated at reflux for 19 h, and then the solvent was removed by
rotary evaporator and the crude product 6a was purified by column
chromatography on silica gel with dichloromethane: gold solid, 81.6
mg, 77%.
DFT Calculations. Geometry optimizations and energy calcu-
lations for the 12 possible cycloplatination products were carried out
using the Gaussian 09 (G09) program18 at density functional theory
level with the M062X functional19 and def2-TZVP basis set for Pt20
and cc-pVDZ21 for other atoms (M062X/def2-TZVP-Pt/cc-pVDZ).
The solvent effects were simulated with the polarizable continuum
model using the integral equation formalism variant (PCM).22 The
frequency calculation was performed for each compound at the
optimized geometry at the same level of theory as used in
optimization. All the computations in this work were completed at
East Carolina University using the Altix 4700 computer cluster.
Reaction of L4 with K2PtCl4 in Acetic Acid: Preparation of 4a. To
a 50 mL round-bottom flask with condenser were added L4 (130 mg,
0.5 mmol), K2PtCl4 (208 mg, 0.5 mmol), and glacial acetic acid (20
mL). The mixture was heated at reflux for 12 h, and then the yellow
precipitate was collected by suction filtration. The crude product was
purified by column chromatography on silica gel with dichloromethane
and ethyl acetate (v/v 50/1): yellow solid, 0.18 g, 73%. 1H NMR (400
ASSOCIATED CONTENT
* Supporting Information
■
S
3
MHz, CDCl3): δ 9.70 (d, J = 5.4, H-6′), 8.10 (d, J = 7.6, JPt−H = 47,
NMR spectra of the ligands and the complexes. A text file of all
computed molecule Cartesian coordinates in .xyz format for
convenient visualization. The Supporting Information is
available free of charge on the ACS Publications website at
DOI: 10.1021/acs.organomet.5b00326.
H-6″), 8.04 (d, J = 7.4, H-4′), 7.97 (t, J = 7.8, H-4), 7.93 (d, J = 8.1, H-
3′), 7.82 (d, J = 7.8, H-3), 7.62 (t, J = 6.4, H-5′), 7.49 (d, J = 8.0, H-5),
7.05 (m, 3 H, phenyl-H), 4.38 (q, J = 7.1, CH), 1.87 (d, J = 7.2, CH3).
13C NMR (75 MHz, DMSO-d6): δ 164.2, 157.9, 155.5, 148.1, 142.5,
140.3, 139.9, 139.0, 130.1, 127.6, 126.7, 125.7, 124.9, 123.9, 123.6,
122.5, 54.5, 28.2.
AUTHOR INFORMATION
Corresponding Authors
*E-mail (Y. Li): liyu@ecu.edu.
*E-mail (S. Huo): huos@ecu.edu.
■
Reaction of L4 with K2PtCl4 in Acetonitrile. To a 50 mL round-
bottom flask with condenser were added L4 (130 mg, 0.5 mmol),
K2PtCl4 (208 mg, 0.5 mmol), and acetonitrile (20 mL). The mixture
was heated at reflux for 2 d, and then the solvent was removed by
rotary evaporator. The crude product 4a was purified by column
chromatography on silica gel with dichloromethane and ethyl acetate
(v/v 50/1): yellow solid, 73.1 mg, 30%.
Notes
The authors declare no competing financial interest.
Reaction of L5 with K2PtCl4 in Acetic Acid: Preparation of 5b. To
a 50 mL round-bottom flask with condenser were added L5 (129 mg,
0.5 mmol), K2PtCl4 (208 mg, 0.5 mmol), and glacial acetic acid (20
mL). The mixture was heated at reflux for 17 h, and then 10 mL of
water was added. The orange precipitate was collected using suction
ACKNOWLEDGMENTS
■
Acknowledgment is made to the Donors of the American
Chemical Society Petroleum Research Fund (#51147-UR3), for
partial support of this research. J.C. is the recipient of a
Burroughs-Wellcome fellowship.
1
filtration, 170 mg, 70%. H NMR (400 MHz, CD2Cl2): δ 8.97 (d, J =
3
5.4, JPt−H = 13.4, H-6′), 8.13 (td, J = 7.9, 1.7, H-4′), 7.97 (d, J = 8.2,
H-3′), 7.95 (s, 2JPt−H = 57, 1 H), 7.75 (m, H-4 and H-5′), 7.55 (d, J =
4
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■
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DOI: 10.1021/acs.organomet.5b00326
Organometallics XXXX, XXX, XXX−XXX